In order to fully understand this invention, the reader should have an appreciation of the action of a saw chain that is cutting through a tree or log. A saw chain is made up of a large number of individual links (100 plus) that range from about a half inch in length to an inch in length. The links are pivotally connected together, mounted to the periphery of an ovally shaped guide bar, driven around the periphery of the bar at speeds in the range of 60 miles per hour and then forced into a log where it cuts into and through knots, twists, knurls and the like common in trees and logs.
Regardless of how tight the chain is mounted on the guide bar, the cutting links within the chain link sequence are jerked and twisted, in and out of the kerf and from side to side (as permitted by the pivotal connections) as the cutters engage and cut through the wood and its variety of irregularities. During this process of cutting, any time that a cutting edge becomes oriented so that it gets buried too deep into the wood, the forward momentum of the chain is brought to an abrupt stop. This can result in "kickback", a term used to describe a rearward thrusting of the guide bar and saw chain toward the operator.
The incidence of kickback has been greatly reduced by dealing with the in and out pivotal action of the cutter. A variety of depth gauge and bumper link configurations have been provided to effectively prevent the cutting edge from diving too deep into the kerf. However, kickback has not been eliminated because such configurations have not considered the side plate cutting edge.
The conventional cutter is L-shaped with the primary cutting edge being on the top plate of the cutter (the lateral leg of the L in an inverted position). It also includes a secondary cutting edge that wraps around the corner of the L and at least partially down the side plate of the cutter. This secondary cutting edge functions to cut the wood fibers along the side walls of the kerf. As mentioned, the saw chain is jerked and twisted sideways as well as in and out of the kerf bottom and should this secondary cutting edge become oriented so as to dive or dig into the side wall of the kerf, kickback can occur. It is the prevention of this sidewall digging in condition to which the present invention is directed.
L-shaped cutters (sometimes referred to as hooded cutters) can be classified in two broad categories based on the manner by which the cutting edges are sharpened. Conventional cutters are sharpened by grinding the surfaces forming the underside or inside surfaces behind the cutting edges. A file is inserted into the space between the depth gauge and cutter (the gullet of the cutting link) and angled under the corner formed by the top and side plates of the cutter. Top sharpening cutters (the secondary category) are configured so as to be sharpened by simply grinding the outside surfaces behind the cutting edges provided on the top and side plates.
Top sharpening saw chain is available as a convenience chain for the reason that its top sharpening capability allows it to be "automatically" sharpened. It is also recognized as being less protectable against kickback as compared to conventional chain. This is largely due to the configuration of the cutter that allows top sharpening. This configuration creates a cutting edge angle that is "hungry" or "highly aggressive" which, increases the tendency of the cutter to dig in and kick back. Accordingly, improvements to reduce kickback have heretofore been initially developed for conventional saw chain. Features that were proven effective for conventional saw chain were then applied to top sharpening saw chain with proportionate reductions in kickback.
In spite of numerous major improvements, kickback has not been eliminated because of the failure to solve the digging in problem of the secondary cutting edge. A depth gauge that is positioned to the side of the cutter for controlling side cutting penetration (an offset depth gauge) has little or no effect on prior cutting link designs because of the spacing that is necessitated by the gullet. Conventional cutting chain requires this spacing in order to provide for the sharpening implement e.g. a file.
A gullet spacing has also been heretofore required for top sharpening chain in order for it to be automatically sharpened (previously the only justification for its existence). Concepts of automatic sharpening dictate that the depth gauge be ground simultaneously with the cutting edge being sharpened which is achieved by spacing the depth gauge forward of the cutting link's center a distance greater than the cutting edge is rearward of that center. These spacings create a gullet that is even greater than that required for conventional cutting links.